Fixing inherited Sinclair ZX81

[mikeyp]

Hi all

I love finding enthusiastic communities where people are willing to help complete strangers. Keeps me feeling like there’s good in the world in these strange times.

I’ve been blessed enough to inherit my Grandpa’s Sinclair Spectrum +3 128k and his ZX81. He’s still alive but has no use for them any more and wants to see my enjoy them. Both machines are cosmetically in lovely condition and the spectrum works flawlessly save needing a new drive belt. Not hard to sort. I’ve tuned it in to an old black and white tv to give me a starting point for the next problem.

The ZX81 is being more troublesome. All I get is a plain black screen when I power it on. All the chips get warm but not hot. The heat sink for the 5V regulator is (painfully) the hottest thing there. I’ve never seen the ZX81 working. Please excuse my Grandpa’s power mod. I guess he thought it was a bad contact and soldered the wires in the hope it would resolve the issue.

Please would you help me resurrect this lovely little machine. Please note that whilst my soldering/desoldering skills are decent and I’m happy to probe with a good multimeter, I have no access to an oscilloscope.

Thank you in advance.

[dessony]

Mikeyp,

Please try to get a zoom-in picture of the ULA area. We would like to know the ULA's version on your ZX81 motherboard.

Thanks,
DesSony

[mikeyp]

It reads

FERRANTI
ULA2C210E
8307

[1024MAK]

So that’s the later ULA and it was made in week 7 of 1983.

Mark

[mikeyp]

So that’s the later ULA and it was made in week 7 of 1983.

Mark

That’s cool to know. So it’s at most 5 years older than me.

[dessony]

That is great! It shows a back porch wave when viewing on an oscilloscope!

DesSony

[mikeyp]

That is great! It shows a back porch wave when viewing on an oscilloscope!

DesSony

It would be great if it worked. I'm reading more and more that the ULA is the most common point of failure.

I'm starting to think I need to desolder it, install a socket and order a vLA81 or similar.

Is there anything else I should check/try first?

[1024MAK]

So the first step is to use a multimeter to test the output voltage of the 7805 voltage regulator. Be very careful that you don’t slip and short anything out. The black (negative) meter lead/probe should go on the metal heatsink tab of the 7805. The red (positive) meter lead/probe should go on the right hand leg of the 7805. The voltage should be between 4.75V and 5.25V.

Mark

[bola_dor]

Are you sure the TV sintonizes the proper channel?
If you have a simple voltimeter check voltages at all integrated circuits, that includes the 7805 regulator that should be about 10v with the computer turned on at the input and 5v +/-10% at the output, you can check the other four ICs in the link at 1024MAK signature..
If you are not sure about the TV I suggest you to make a simple transistor composite mod that can be clamped with crocodiles to the modulator leads and GND just to have one less thing to think about during diagnosis.
Sorted this . I see all the IC are soldered so you can't easily exchange them to test with another ones.
Do you have a rampack? Using one known as good will rule out the ram as is will override de internal ram completely.
Then you can check for continuity and shorts all the D lines and A lines between ULA and Z80 and rom and RAM. Search for the schematics on line. Some of the buses have 470ohm resistors and others 1Kohm ( you won't see dirext continuity but the corresponding resistance).
All this can be done without any complicated instruments.. then you will need an oscilloscope or at least a logic probe.
There are little things to test only with a comon multimeter and with not much reliability as some of the Z80 control lines not being dead or the reset working

[bola_dor]

That is great! It shows a back porch wave when viewing on an oscilloscope!

DesSony

It would be great if it worked. I'm reading more and more that the ULA is the most common point of failure.

I'm starting to think I need to desolder it, install a socket and order a vLA81 or similar.

Is there anything else I should check/try first?

Desoldering the ULA is not an easy task and can be harmful to the board. I will rule out any other chance before trying this. With no image it can be almost anything Z80 or ULA probably but anything may be wrong.